Market demand continues for new polypropylene products to meet ever-expanding applications. To meet demand, it is known that the selection of comonomer(s) for copolymerization with propylene allows for wide modification in resin properties such as hardness, tensile strength, stiffness, density, melting point, impact strength, etc.
In particular, propylene-based polymer which includes one, two, or more comonomers (such as ethylene and/or butene) and hereafter referred to as “propylene interpolymer” is finding increased commercial interest in applications such as films, packaging, and fibers. However, obstacles exist when producing propylene-based polymer with butene comonomer with conventional “commercial-scale” (40 kilo-tons annually (KTA) or larger or 5 metric tons/hr) gas-phase polymerization equipment.
In conventional commercial-scale gas-phase polymerization of propylene, propane naturally accumulates in the reactor system and synergistically functions as a condensing agent. When propylene-based polymer with one or more comonomer is produced, less propylene is fed to the gas-phase polymerization reactor when compared to propylene homopolymer production. As a result, the rate at which propane accumulates in the reactor system is slow, typically on the order of one or more days. In order to have somewhat equivalent gas density and cycle gas dew point needed to arrive at high production rate for propylene terpolymer, more propane must accumulate in the reactor than with propylene homopolymer production.
The slow naturally occurring build-up of propane in commercial-scale propylene interpolymer production where one or more comonomers are added to propylene has several drawbacks. With insufficient propane (i.e., insufficient condensing agent) in the reactor system, condensing mode operation cannot be achieved during the course of a normal run. Operating the gas-phase polymerization reactor in dry mode (no condensing mode) requires very low reaction temperature and results in prohibitively low production rates for commercial-scale.
Attempts have been made to add an inert component (often called “induced condensing agent” such as nitrogen or an alkane other than propane) on a continuous basis in order to adjust and maintain the partial pressures of the components in the reaction system to acceptable levels for commercial-scale propylene terpolymer production. Inert component addition adds to material costs and equipment costs, complicates reactor control, and achieves, at best, 60% of nameplate capacity. Inert component addition, therefore is not practical for the production of propylene terpolymer on conventional commercial-scale gas-phase polymerization reactors.
A need exists for a system and method to produce propylene-based polymer with one or more comonomers at nameplate rates in commercial-scale production using conventional gas-phase polymerization reactors. A need further exists for a system and method for reducing the transition time from commercial scale propylene homopolymer production to commercial-scale propylene interpolymer production.